Drive south down Camp Williams Road, a highway outside Salt Lake City, and your eye is drawn to the left. A gun-mounted helicopter and other military hardware marks the entrance of the Utah army national guard base. The ice-capped Rockies soar in the distance.

To the right there is little to see: featureless scrubland, a metal fence, some warehouses. A small exit – not marked on ordinary maps – takes you up a curving road. A yellow sign says this is military property closed to unauthorised personnel.

Further up the hill, invisible from the highway, you encounter concrete walls, a security boom and checkpoint with guards, sniffer dogs and cameras.

Two plaques with official seals announce the presence of the office of the director of national intelligence and the National Security Agency.